1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a semiconductor device having a vertical Hall element to detect a magnetic field in a horizontal direction.
2. Description of the Related Art
A Hall element is capable of detecting position or angle as a magnetic sensor without contacting to an object and accordingly has various uses. Magnetic sensors that use a horizontal Hall element for detection of a magnetic field component perpendicular to a semiconductor substrate surface are particularly well known. There have been also proposed various magnetic sensors that use a vertical Hall element for detection of a magnetic field component parallel to a substrate surface. Proposed magnetic sensors further include a magnetic sensor that combines a horizontal Hall element and a vertical Hall element to detect a magnetic field two-dimensionally or three-dimensionally.
Enhancement of sensitivity, however, is more difficult in a vertical Hall element than in a horizontal Hall element.
In Japanese Patent Application Laid-open No. 2005-333103 (see, particularly, FIG. 3), there is proposed a configuration in which an electrode formed of an N-type diffusion layer and an electrode isolation diffusion layer (P well) for isolation of adjacent electrodes from each other are arranged in a magnetism sensing portion (N well) formed in a P-type substrate, and the impurity concentration of the magnetism sensing portion is distributed so as to be highest on a front surface of the substrate and to decrease gradually as the depth from the front surface increases. In the above-mentioned configuration the width of a depletion layer formed and the width of the electrode isolation diffusion layer which becomes narrower as the depth from the substrate's front surface increases complement each other so as to keep the flow of current from spreading in the magnetism sensing portion, thereby relatively increasing a current component that flows in a direction perpendicular to the substrate and improving sensitivity.
The structure disclosed in Japanese Patent Application Laid-open No. 2005-333103 has the following characteristics.
Specifically, a current supplied between two electrodes that sandwich the electrode isolation diffusion layer flows from one of the electrodes on the front surface of the substrate in a direction toward the rear surface of the substrate (downward), then flows in a direction parallel to the substrate in a part below the electrode isolation diffusion layer, and, from there, flows to the other of the two electrodes on the front surface of the substrate (upward). The current flowing in the direction parallel to the substrate in the part below the electrode isolation diffusion layer concentrates particularly on a region that is immediately below the electrode isolation diffusion layer and that has the lowest resistance (highest concentration) throughout the part of the magnetism sensing portion that is located below the electrode isolation diffusion layer. Since the resistance of the magnetism sensing portion increases toward the rear surface side of the substrate, substantially no current flows in a region of the magnetism sensing portion close to the rear surface of the substrate and locating below the electrode isolation diffusion layer. The flow of current in the direction parallel to the substrate consequently has a narrow width in the depth direction of the substrate.
The magnetic sensitivity of a Hall element is known to increase in proportion to the width of the flow of current. The width of the flow of current in the direction parallel to the substrate is narrow as described above in the structure disclosed in Japanese Patent Application Laid-open No. 2005-333103, resulting in a difficulty in improving sensitivity.
In addition, since in the structure disclosed in Japanese Patent Application Laid-open No. 2005-333103 the current flows in a region that has an uneven concentration distribution, variation in current path occurs, leading to an increase in offset voltage.